1.1 Technical Field
The present invention relates to a stable aqueous solution (A) comprising an azo dye, a borate buffer and one or more masking agents, wherein the azo dye changes its coloration or coloration intensity in the presence of chlorine dioxide. The present invention further relates to a process for manufacturing the stable aqueous azo-dye solution (A), and to its use for the determination of residual chlorine dioxide in water.
1.2 Background of Art
Since the discovery of the interactions between chlorine and the microorganisms present in untreated water and the resulting formation of toxic compounds, such as trihalomethanes, many studies have been carried out worldwide in order to find replacement solutions for disinfecting drinking water. Among the disinfectants proposed is chlorine dioxide. Accordingly, in recent decades, chlorine dioxide has been used in many countries to disinfect drinking waters and to condition industrial waters.
Since a certain amount of disinfectant must be present in the treated water to prevent it from being recontaminated, it is essential to determine accurately the residual amount of the disinfectant in the water.
Moreover, during the treatment of water and the period in the distribution network, chlorine dioxide participates in various oxidation reactions which lead to the formation of reduction/decomposition by-products, mainly consisting of chlorides, chlorates and chlorates. Thus, it is necessary to have a reliable method for determining the chlorine dioxide content in the treated water even in the presence of other oxidizing agents and chloro-compounds.
Lastly, the process for determining the chlorine dioxide content in water should comprise a limited number of operations and should be able to be carried out directly on site so as to avoid a loss of chlorine dioxide by degassing.
Although several methods have been proposed for determining chlorine dioxide, none of them satisfy all the above-mentioned criteria. Various methods, particularly colorimetric methods, available to date are listed in the thesis by J. D. Peak, Edmonton, Alberta, 1991. According to J. D. Peak, because of their virtually unselective natures, these colorimetric methods have been excluded from routine practices in industry.
Specifically, the method using DPD (N,N-diethyl-p-phenylenediamine sulphate), which is not sufficiently selective, often leads to erroneous results. Furthermore, it cannot determine the chlorine dioxide contents less than 0.1 mg/l.
Similarly, the method based on the decolorization of Alizarin Violet 3R (ACVK), developed by W. J. Masschelein (Analytical Chemistry, 38:1839, 1996) has a quantification threshold greater than 0.1 mg/l of chlorine dioxide.
To make the Chlorophenol Red (CPR) method selective, J. Fletcher and P. Hemmings (Analyst, 110:695, 1985) have proposed to use masking agents. This method, which involves several steps, consists of mixing the sample with a sodium cyclamate solution, adding immediately a buffer solution with stirring, followed by a chlorophenol red solution, and finally adding a thioacetamide solution. By measuring the absorbance of the final mixture at 520 nm using a UV-visible spectrophotometer, the residual content of chlorine dioxide in the sample can be determined.
However, the major drawback of this method is that it involves many steps of placing the sample in contact with a series of reagents and thus leads to a considerable and uncontrolled loss of chlorine dioxide by degassing (up to 30%).
The present invention by the applicants provides an aqueous solution (A) comprising an azo dye, a borate buffer and one or more masking agents, wherein the azo dye changes its coloration or coloration intensity in the presence of chlorine dioxide, and now makes it possible to determine accurately and selectively the residual content of chlorine dioxide in water, in particular, in drinking water.
The azo dye is advantageously chosen from amaranth (trisodium salt of 1-(4-sulfo-1-naphthylazo)-2-naphthol-3,6-disulfonic acid, C20H11N2Na3O10S3), C.I. 16185, and Evans blue (tetrasodium salt of 6,6xe2x80x2-[(3,3xe2x80x2-dimethyl[1,1xe2x80x2-biphenyl]-4,4xe2x80x2-diyl)bis(azo)]bis[4-amino-5-hydroxy-1,3-naphthalenedisulfonic acid, C34H24N6Na4O14S4], C.I. 23860.
The concentration of azo dye in solution (A) is generally between about 1xc3x9710xe2x88x926 and about 1xc3x9710xe2x88x923 M. It is preferably between about 2xc3x9710xe2x88x925 and about 8xc3x9710xe2x88x924 M. An amaranth concentration of about 2xc3x9710xe2x88x924 M has been found to be particularly advantageous. When the dye is Evans blue, a concentration of about 5xc3x9710xe2x88x925 M is advantageously chosen.
The borate ion is generally present in solution (A) in a proportion of from about 5xc3x9710xe2x88x923 to about 1xc3x9710xe2x88x921 M. A borate ion concentration of about 5xc3x9710xe2x88x922 M is preferred.
In this specification, the term xe2x80x9cmasking agentxe2x80x9d means any compound capable of reacting with free chlorine: for example, glycine, cyclamate of alkali metal or alkaline earth metal, and aqueous ammonia. The amount of masking agent(s) used in solution (A) depends on its (their) nature. Preferably, aqueous ammonia is used, and in an amount advantageously between about 1 and about 4 g/l of solution (A).
The aqueous azo-dye solution according to the present invention can further comprise one or more metal-chelating agents such as EDTA (ethylenediaminetetra-acetic acid) salts. The amount of chelating agent(s) used varies depending on its (their) nature. In the case of the sodium salt of EDTA, the amount used per liter of solution (A) is generally between about 0.5 and about 2 g and preferably about 1 g. The solution (A) which is most particularly suitable contains, per liter, about 5xc3x9710xe2x88x922 mol of borate, about 1.5xc3x9710xe2x88x922 mol of aqueous ammonia, about 1 g of sodium salt of EDTA, and about 2xc3x9710xe2x88x924 mol of amaranth or about 5xc3x9710xe2x88x925 mol of Evans blue.
Another subject of the invention is a process for manufacturing a solution (A). In general, this process comprises following steps:
(a) a buffered aqueous azo-dye solution is prepared by introducing the azo dye, the masking agent(s) and the borate buffer solution into a container containing a sufficient amount of double-deionized water;
(b) the chelating agent dissolved in advance in double-deionized water is optionally added thereto, with stirring; and
(c) the solution is made up to the desired volume with double-deionized water.
A pH of the aqueous azo-dye solution prepared in step (a), should be preferably about 9.2.
More particularly, the process for manufacturing a solution (A) comprises the following successive steps:
(i) the azo dye is dissolved in double-deionized water in a container;
(ii) a borate buffer solution is then introduced therein, followed by a solution of masking agent(s);
(iii) double-deionized water is added thereto and the pH is measured;
(iv) the pH is adjusted to about 9.2, if necessary;
(v) the chelating agent is optionally dissolved, with stirring; and
(vi) the solution is made up to the desired volume with double-deionized water.
Advantageously, an aqueous ammonia solution is used to adjust the pH. A concentrated aqueous ammonia solution at about 28% by weight is particularly suitable to be used for pH adjustment and as a masking agent.
The aqueous azo-dye solution (A) thus prepared remains stable at room temperature for at least one month in a closed bottle.
Yet another subject of the present invention is a process for determining the residual chlorine dioxide content in an industrial water or drinking water after biocidal treatment or disinfection and in distribution circuits. This process consists of placing the water to be analyzed in contact with the aqueous solution (A) and then measuring the absorbance of the resultant solution (S) using a UV-visible spectrophotometer, at the specific wavelength for the azo dye chosen. This is 521 nm in the case of amaranth and 606 nm in the case of Evans blue.
This operation of placing two solutions in contact is generally carried out in the volume ratio: the water to be analyzed (water sample)/the aqueous solution (A), to be between about 10 and about 30 and preferably about 24.
As a reference for the absorbance measurement, the water sample itself to which a sufficient amount of a reducing agent is added, is used. As a reducing agent, an agent which particularly reduces chlorine dioxide, such as sodium thiosulphate, is preferable.
Advantageously, the water sample is placed in contact with solution (A) containing, per liter, about 5xc3x9710xe2x88x922 mol of borate, about 1.5xc3x9710xe2x88x922 mol of aqueous ammonia, about 1 g of sodium salt of EDTA and about 2xc3x9710xe2x88x924 mol of amaranth or about 5xc3x9710xe2x88x925 mol of Evans blue.
Preferably, the water sample is taken directly from the source and the operation of placing it in contact with the solution (A) is carried out by dipping the outlet of the water sample container directly into the solution (A). Working in this way makes it possible to avoid any loss of chlorine dioxide by degassing and to minimize errors arising from the sample transferring.
The absorbance of the resultant solution at the specific wavelength for the azo dye chosen is then measured in a quartz cuvette with an optical path length of 2.5 cm, using a UV-visible spectrophotometer and, as a reference, the water sample to which sodium thiosulphate has been added.
By plotting the absorbance measurement on a calibration curve relative to the reference, the residual chlorine dioxide content in the water sample is obtained. The calibration curve is generally pre-established by a common method based on known concentrations of chlorine dioxide solutions, and only the linear part of the calibration curve (i.e., less than 500 xcexcg of ClO2 per liter) is used.
The determination process can be readily adapted to spectrophotometers equipped with different optical path lengths (1, 5 or 10 cm) by adjusting the concentration of the dye in solution (A) and by establishing the corresponding calibration curve.
The process according to the present invention thus makes it possible to selectively determine the residual chlorine dioxide content in drinking water or industrial water, down to a concentration as low as 6 xcexcg/l. Furthermore, after placing the aqueous solution (A) in contact with the water sample, the UV-visible spectrophotometric measurement can be made even 7 to 10 days later.